Wireless communication increasingly uses higher-order modulations such as 256QAM modulation scheme to improve spectral efficiency. This requires lower distortion (e.g., lower error vector magnitude and lower spectral regrowth) from all radio circuit blocks, including the transmitter power amplifier and the power amplifier driver that precedes the transmitter power amplifier. Making power amplifier stages both highly linear and power efficient is not an easy task, particularly in CMOS technology. Therefore, various linearization techniques are used to achieve linearity, among which digital predistortion (DPD) technique is generally considered a desired technique. There are a variety of other techniques that are considered reasonable, for example, feedforward, analog predistortion, indirect (e.g., Cartesian frequency translation, envelope, etc.) feedback.
DPD technique is the most widely adopted linearization technique. However, DPD requires widening the transmitter baseband filter bandwidth (e.g. from 9 MHz to 60 MHz for WLAN) to pass the distortion-cancelling energy, and increasing the peak-to-average ratio of the signal in the transmit (TX) chain to cancel the PA compression. Both of these effects increase the noise contribution from baseband blocks such as a digital-to analog (DAC) block, which may leak into adjacent cellular bands.